Imagine a deck of cards lying flat on a table. Now, pretend those cards are layers of rock, and tilt them up on one side. The cards that begin to slide first are layers of clay, which, unfortunately for homeowners and drivers near San Diego today, are also the weakest.

That kind of analogous disaster is the worst-case scenario when it comes to landslides, according to Larry Gurrola, a California-based geological consultant who specializes in landslides and fault hazards. Officials are still looking for answers after the earth sunk out from underneath a highway and several mansions in the La Jolla this morning, destroying two homes and leaving at least a dozen others uninhabitable. But leaky water mains and lawn irrigation are the most likely cuplrits, says Ed Harp, a geologist for the U.S. Geological Survey.

No one knows just how many homes stand on slide-susceptible infrastructure, but the absence of building regulations during the post-World War II housing boom doesn't help. Landslides have been a problem in the area since 1961, but "given the value of land prices, it gives people with houses short memories," Gurrola says. "It wasn't until the late 60s, early 70s, when geologic investigations were part of development."

Old homes were grandfathered into the new guidelines. Today, when builders survey for areas in danger of landslides, they examine aerial photos to look at the texture and vegetation of the landscape. Landslide-ridden territory typically sports somewhat random water springs and peculiar vegetation. The rocks tend to be weaker, and although the ground might be stable for hundreds if not thousands of years, home construction and the installation of water and septic pipes can easily trigger a new slide. "The landslide could be a thousand years old or even 40,000 years old," Gurrola says. "But it's still a concern because a slide might have loosened the rocks around it, making them weaker and fractured."

If an old landslide is identified, builders are required to investigate and conduct vigorous exercises on the slopes before any building can take place. In addition, they have to make sure that in the event of an earthquake, the ground won't just suddenly destabilize, Gurrola says. ­­­—Wayne Ma